"There's no mystery" about the collapse of a third huge skyscraper at the World Trade Center on 9/11, the lead investigator of the National Institute of Standards and Technology, told me last week.
After three years extensive and exhaustive investigation NIST has published their final report on the World Trade Center and concluded that fire caused Tower 7 to collapse (pdf link). And they deduced from a series of highly complex computer models that the collapse started with the failure of just one column, column 79 on the 13th floor.
Dr Sunder added that the collapse of WTC7 represents the first known instance of the total collapse of a tall building primarily due to fires.
Why was it the first in the world? NIST scientists say it was because fires were allowed to burn in Tower 7 for nearly seven hours and the mains water had been severed by the collapse of WTC1, meaning that fire fighters were unable to fight the fires. The failure of the mains water supply also cut the sprinkler system to the lower half of the building, where the fires burnt for longer and more fiercely.
NIST specifically looked at whether controlled demolition using conventional explosives could have caused the collapse. But they found that the minimum charge necessary to destroy just one key column would have produced a huge amount of noise - between 130-140 Db even one kilometre away from WTC7 - the equivalent of standing next to a jet plane engine.
Yet NIST found no witnesses who had heard anything like that, nor any such noise on any of the videos of the collapse and none of the shattered windows that would have been expected on the backs of buildings.
Dr Sunder told me that they judged other possible hypotheses, such as the theory that there was a controlled demolition using an incendiary called thermite or thermate, were "not credible enough to justify investigation".
Yesterday was the closing day for comments on that final official report.
We have been filming a new set of interviews for an update of "The Conspiracy Files: 9/11 - The Third Tower" to be broadcast on BBC Two soon. With the publication of the final official report on 9/11, officials think the long process of investigation is at last nearly over.